Enhanced magnetic field communication system

ABSTRACT

Devices and methods are provided to enhance magnetic field communication. One aspect of the present subject matter relates to a method for transmitting and receiving signals using an antenna element electrically connected to a driver and an amplifier. According to various embodiments of the method, a first signal is transmitted from the antenna element and a second signal that was induced in the antenna element is received. Transmitting the first signal includes driving the first signal through the antenna element using the driver, and monitoring the first signal through an input impedance of the amplifier. Receiving the second signal includes reducing the input impedance of the amplifier, and receiving the second signal at the amplifier through the reduced input impedance. Various embodiments shield the antenna element from electric and electromagnetic fields. Other aspects are provided herein.

RELATED APPLICATION

This application is a divisional under 37 C.F.R. 1.53(b) of U.S. patentapplication Ser. No. 11/165,142 filed Jun. 23, 2005, which is adivisional under 37 C.F.R. 1.53(b) of U.S. patent application Ser. No.10/722,093 filed Nov. 25, 2003, which applications are incorporatedherein by reference and made a part hereof.

TECHNICAL FIELD

This application relates generally to communication systems, and, moreparticularly, to systems, devices and methods to receive and transmitmagnetic field communication signals.

BACKGROUND

The dominant mode of signal propagation in some known communicationssystems that have relatively large distances between the receiving andtransmitting antennas is electromagnetic fields. Due to their ability totravel relatively long distances, noise attributed to electromagneticfields can be a significant problem. Known communication antenna designsattempt to optimize the pickup of electromagnetic, electric and magneticfields, but few attempts are made to reduce the affects of propagatedand conducted electromagnetic noise, propagated and conducted electricfield noise, and propagated and conducted magnetic field noise.

For short-range communication applications, the antenna must have goodnear field performance and should reject unwanted signals and/or noisefrom unwanted radiating and conducting sources. Examples of suchshort-range communication applications include, but are not limited to,hearing aid to telephone receiver communications, hearing aid to hearingaid communications, and programmer to hearing aid communications.Examples of unwanted radiating sources include radio and televisionstations, and the like. Examples of unintentional radiating sources ofunwanted electromagnetic interference (EMI) include computers,televisions, electric motors and the like. Examples of conductedinterference sources are electric field and electromagnetic radiationconducted via electrically conductive objects such as metal, human skinand conductive liquids significantly impairing communications.

There are other design challenges facing short range communicationssystems. Virtually all short-range communication systems are compact,battery or RF powered and low-cost. Because of the previousrequirements, most all short range communication systems are half-duplexor simplex communication systems that are capable of transmitting datain only one direction at a time. Known half-duplex or simplexcommunications system use electrical or mechanical switches to changebetween receive and transmit modes. These switches require a significantamount of time to switch between modes. Other problems include theadditional parts and separate control lines to select between thereceive and transmit modes.

The communication system is tuned to transmit and receive communicationsignals at a desired resonant frequency. However, the equivalentparallel parasitic capacitance of the coil and the capacitance of thelow noise amplifier and other portions of the circuit can detrimentallyand unpredictably affect the resonant frequency of the communicationsystem.

There is a need in the art to provide improved communication systems andmethods for transmitting and receiving short range data.

SUMMARY

The above-mentioned problems are addressed by the present subject matterand will be understood by reading and studying the followingspecification. Various aspects and embodiments of the present subjectmatter provide low noise amplifier and antenna designs that enhancemagnetic field communications and minimize interference. Embodiments ofthe present subject matter have a number of advantages, including butnot limited to: reducing common mode EMI pickup by using adifferentially-driven receiver circuit; eliminating the control line toswitch between transmit and receive modes; providing an adjustablecommunication bandwidth for the antenna; diminishing DC offset biasissues, reducing the RF voltages presented to the receive and transmitcircuits; reducing the time to switch between transmitting and receivingmodes; providing an integrated electrostatic shielding for the antenna;providing at least two selectable RF power levels; and reducing theeffect of parasitic capacitance on the resonant frequency of thecommunication system.

One aspect of the present subject matter relates to a communicationcircuit to receive and transmit signals. According to variousembodiments, the circuit includes an antenna element and anelectrostatic conductor. The antenna element has a first terminal and asecond terminal. The electrostatic conductor is positioned to shield theantenna element from electric fields. The antenna element is adapted toinduce a received signal at the first and second terminals when theantenna element is in a magnetic field. The circuit also includes adriver, a differential amplifier and a switch. The driver is connectedto at least one of the first and second terminals to energize theantenna element with a transmitted signal. The differential amplifierhas a first input connected to the first terminal of the antenna elementand a second input connected to the second terminal of the antennaelement. The differential amplifier has a selectable input impedance. Alower first input impedance is selected to amplify the received signalfrom the antenna element, and a higher second input impedance isselected to monitor the transmitted signal from the driver to providethe ability to perform self alignment and self diagnostic processes onthe communication circuit. The switch toggles an effective inputimpedance for the differential amplifier between the second impedanceand the first impedance.

According to various embodiments, the circuit includes an antennaelement, an amplifier circuit, a driver circuit and a control line. Theantenna element includes a first and a second terminal, an inductivecoil electrically connected to the first and the second terminals, andan electrostatic conductor to shield the inductive coil against electricfields. The amplifier circuit is adapted to amplify amagnetically-induced signal received by the antenna element. Theamplifier circuit includes a differential amplifier, a first inputimpedance, a second input impedance, a predetermined feedback impedanceand an impedance shunt. The differential amplifier includes a firstinput, a second input and an output. The first input impedance isconnected between the first input of the amplifier and the firstterminal of the antenna element. The second input impedance is connectedbetween the second input of the amplifier and the second terminal of theantenna element. Each of the first input impedance and the second inputimpedance includes a first element and a second element. Thepredetermined feedback impedance is connected between the output and atleast one of the two inputs of the differential amplifier. The inputimpedance shunt is connected across the second element for each of thetwo inputs of the differential amplifier. The driver circuit is adaptedto drive the antenna element with a transmission signal. The controlline is connected to the input impedance shunt to selectively shunt thesecond element for each of the two inputs of the amplifier. The controlline is used to selectively reduce an effective input impedance to thedifferential amplifier to receive the magnetically-induced signalreceived by the antenna element.

One aspect of the present subject matter relates to a method fortransmitting and receiving signals using an antenna element electricallyconnected to a driver and an amplifier. According to various embodimentsof the method, a first signal is transmitted from the antenna elementand a second signal that was induced in the antenna element is received.Transmitting the first signal includes driving the first signal throughthe antenna element using the driver, and monitoring the first signalthrough an input impedance of the amplifier. Receiving the second signalincludes reducing the input impedance of the amplifier, and receivingthe second signal at the amplifier through the reduced input impedance.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects will be apparent to persons skilled in the art upon reading andunderstanding the following detailed description and viewing thedrawings that form a part thereof, each of which are not to be taken ina limiting sense. The scope of the present invention is defined by theappended claims and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a resonant circuit for an antenna element accordingto various embodiments of the present subject matter.

FIG. 2 illustrates a coil for an antenna element according to variousembodiments of the present subject matter.

FIG. 3 illustrates an antenna element according to various embodimentsof the present subject matter.

FIG. 4 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter.

FIG. 5 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter.

FIG. 6 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter.

DETAILED DESCRIPTION

The following detailed description of the present subject matter refersto the accompanying drawings which show, by way of illustration,specific aspects and embodiments in which the present subject matter maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the present subject matter.Other embodiments may be utilized and structural, logical, andelectrical changes may be made without departing from the scope of thepresent subject matter. References to “an”, “one”, or “various”embodiments in this disclosure are not necessarily to the sameembodiment, and such references contemplate more than one embodiment.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present subject matter is definedonly by the appended claims, along with the full scope of legalequivalents to which such claims are entitled.

FIG. 1 illustrates a resonant circuit for an antenna element accordingto various embodiments of the present subject matter. The illustratedcircuit 100 includes an antenna element 102 and a low noise amplifier104. The antenna element 102 includes an inductive coil 106 connected inseries with a tuning capacitor 108. The inductive coil 106 isillustrated with its equivalent parallel parasitic capacitance 110 andtwo coil portions 112A and 112B. As discussed below, the inductive coilhas an electric field shield 114. The capacitive nature of this shield114 contributes to the parasitic capacitance of the circuit. In variousembodiments of the present subject matter, an additional capacitor 116is connected in series with the coil 112A and 112B. This additionalcapacitor 116 makes the resonant circuit less susceptible to parasiticcapacitance of the resonant circuit, and allows a large tuning capacitorto 108 be used to provide the circuit with a desired resonant frequency.A node where the capacitor connects to the coil is a high impedancenode. In receive mode, this high impedance node is particularlysusceptible to picking up electric field radiation, and in transmit modeit generates unacceptable high standing voltage, possibly damaging othernear by circuits.

FIG. 2 illustrates a coil for an antenna element according to variousembodiments of the present subject matter. The illustrated coil 202includes a core 218 having a high magnetic permeability surrounded bycoiled wire 220. Various embodiments include a ferrite core 218. In theillustration, the coiled wire is covered by an insulator 222, which iscovered by an electrostatic conductor 224. One example of anelectrostatic conductor is copper. The figure illustrates copper tapewrapped around the insulator. The copper tape functions as an electricfield shield. In the presence of an electric field, the electric fieldshield collects a surface charge generated by the electric field.

In various embodiments such as illustrated in FIG. 1, the coiled wire220 is split into a first portion 112A and a second portion 112B. Theadditional capacitor 116 is connected in series between the firstportion and the second portion of the coil such that the additionalcapacitor, the first and second portions of the coil, and the nodesbetween the capacitor and the first and second portions of the coil areshielded from electric fields by the electrostatic conductor.

FIG. 3 illustrates an antenna element 302 according to variousembodiments of the present subject matter. In the illustration, a wireconductor 320 is coiled around a core 318 having a high magneticpermeability and high electrical resistivity. In various embodiments,the core includes ferrite. In various embodiments, the core materialincludes air. Other embodiments include other core materials. Asgenerally shown in the figure, a received magnetic field signal 326induces a current signal 328 in the coil 320 and a transmitted currentsignal 328 in the coil 320 induces a magnetic field signal 326. Theillustrated antenna element 302 has an electrostatic conductor 314electrically insulated from the coil 320. In the presence of an electricfield, the electrostatic conductor collects a surface charge generatedby electric field. Thus, the electrostatic conductor functions as ashield against electric fields. In various embodiments, theelectrostatic conductor is formed as a strip that extends across thelength of the coil, as shown in FIG. 3. In various embodiments, theelectrostatic conductor is formed as a cylindrical enclosure, such asshown in FIG. 2, for example.

In various embodiments, the electric field shield conductor 314 is leftfloating. Electrostatic surface charge collects on the conductor.Because like electrical charges naturally repel from each other, theelectrostatic surface charges are distributed across the conductor.Thus, when the electrostatic conductor is left floating, the voltageattributable to the electric fields is uniformly (or approximatelyequally) applied to each terminal 330A and 330B of the coil 320. As willbe described in more detail below, the equal noise signal at eachterminal of the coil is rejected as common mode noise by a low noisedifferential amplifier.

In various embodiments, the electric field shield conductor 314 isgrounded or otherwise connected to a reference voltage such that thesurface charges attributed to the electric fields are removed from theelectrostatic conductor. Thus, neither one of the coil terminals 330Aand 330B is significantly affected by a voltage on the electrostaticconductor.

The signal induced in the coil is received and amplified by a low noiseamplifier. In the above-described embodiments, the electric fields donot significantly contribute to the amplified signal. The electrostaticconductor, in combination with a differential amplifier in variousembodiments, rejects unwanted electric field signals and/or noise.Therefore, the amplified signal is attributable to the magnetic fieldsat the antenna element. The circuitry of the present subject matterrejects electric field signals which can travel relatively longdistances, and can be a source of interfering noise. Thus, the presentsubject matter provides good short-range communication using onlymagnetic field signals.

FIG. 4 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter. FIGS. 5 and 6 may be useful to assist with thediscussion of FIG. 4. The illustrated communication circuit 432 includesan antenna element 402, an amplifier circuit 404, and a transmit drivecircuit 434. The figure also illustrates signal processing circuit 436.The signal processing circuit 436 includes a processor 438 tocommunicate with a receiver circuit 440 to receive an RF output signal442 from the low noise differential amplifier 404 that is representativeof a received signal at the antenna element 402. The processor 438 alsocommunicates with a transmitter circuit 444 to send an RF input signal446 to the transmit drive circuit 434, which in turn energizes theantenna element 404 with a signal representative of the RF input signal446.

The transmit drive circuit 434 functions to energize the antenna element402 with a signal for transmission at a resonant frequency. Theamplifier circuit 404 functions to amplify a signal in preparation forfurther signal processing of the signal. The amplifier circuit 404 isable to receive and amplify a magnetically induced signal received bythe antenna element at a resonant frequency. Additionally, in variousembodiments, the amplifier circuit 404 is able to receive and amplify anenergizing signal transmitted by the transmit drive circuit 434 to theantenna element 402 such that the energizing signal can be monitored andallow self-alignment, tuning optimization, channel selection and selfdiagnostics. The antenna element 402, the transmit drive circuit 434,and the amplifier circuit 404 are discussed in more detail below.

In various embodiments, the antenna element 402 includes an inductivecoil in series with a capacitor. The antenna element is most efficientwhen the antenna elements resonant frequency coincide with the desiredcommunication frequency. The antenna element's resonant frequency isinfluenced by the inductance and capacitance of the antenna. Parasiticcapacitance in the circuit also can influence the resonant frequency.The received magnetic field induces a current in the inductive coil andis forwarded to the low noise differential amplifier.

The illustrated antenna element 402 includes an electric field shield414. In various embodiments, an electrostatic conductor functions as ashield for electric fields. The electrostatic conductor is insulatedfrom windings of the coil, and thus does not shunt the magnetic field.As discussed above, the electrostatic conductor conducts electrostaticsurface charge attributed to electric fields. In various embodiments,the electrostatic conductor is connected to a reference potential (e.g.ground) such that the electrostatic energy is conducted away from thesystem antenna such that the electric field does not significantlyinfluence the signal induced in the coil. In these embodiments, theamplifier 404 need not be a differential amplifier since theelectrostatic energy is removed. In various embodiments, theelectrostatic conductor floats (the conductor is not connected to areference potential), and thus functions as an electrostatic equalizer.When the electrostatic conductor functions as an electrostaticequalizer, the surface charge attributed to electric fields is evenlydistributed throughout the electrostatic conductor such that eachterminal of the coil is equally affected by the electrostatic charge.The terminals of the coil are connected to a differential amplifier 404.The differential input of the amplifier rejects the common mode voltage,including voltage contributed by the electrostatic charge distributedacross the electrostatic conductor.

In various embodiments, the amplifier 404 is a low noise differentialamplifier. Various embodiments of the present subject matter include alow noise voltage-driven operational amplifier, and various embodimentsinclude a low noise current-driven operational amplifier. Voltage-drivenoperational amplifiers and current-driven operational amplifiers areknown to those skilled in the art. The band-pass response of the antennacircuit, and the amplifier's sensitivity to unwanted electric fields arecapable of being modified by adjusting an impedance in the amplifier404. The illustrated low noise differential amplifier includes animpedance switch 448, which is used to adjust an impedance of theamplifier 404 to receive a signal induced in the antenna element 402 bya magnetic field. In various embodiments, for example, the impedanceswitch 448 toggles an effective input impedance of the amplifier betweena larger resistance and a smaller resistance.

There are number of ways to adjust the impedance of the amplifier toachieve a desired band-pass response and gain. Various embodiments ofthe amplifier include various arrangements of various elements thatfunction as input and feedback impedances. Additionally, variousembodiments implement an impedance switch with these various elementsand configurations to adjust the band-pass response and/or gain of theamplifier as desired by actuating the switch.

In various embodiments, the input impedance of each input to thedifferential amplifier 404 includes a first element connected in serieswith a second element. The impedance switch includes a transistor that,when actuated, forms a shunt across the second element to change theeffective input impedance. Thus, in embodiments where the first andsecond elements are in series and the second element is selectivelyshunted, a lower first input impedance (first impedance) is formed bythe first element and a lower second input impedance (second impedance)is formed by a combination of the first element (first impedance) andthe second element (third impedance). In various embodiments, the inputimpedance of each input to the differential amplifier includes a firstelement connected in parallel with a second element. The impedanceswitch includes a transistor that, when actuated, forms a shunt acrossthe second element to change the effective input impedance. Otherswitchable impedance networks fall within the scope of the presentsubject, regardless of whether the switchable impedance network providesan adjustable input impedance or an adjustable feedback impedance, orwhether the switchable impedance network switches between distinct highimpedance and low impedance paths for the input impedance and/or thefeedback impedance.

The transmit drive circuit 434 includes a transmitter driver 450, andcontrol circuitry 452 to control the input impedance of the amplifierand to enable the transmitter driver based on whether a carrier signalis detected. The illustrated control circuitry includes one RF input 454to receive an RF input signal 446, and one RF output 456 to transmit acorresponding RF signal 458 to the driver. At least one control output460 is used to control the impedance switch to appropriately control theimpedance of the amplifier, and to enable the transmitter drive stagevia a transmit (XMIT) enable signal.

In various embodiments, when the RF input 454 has a carrier of adequateRF drive level, the control circuitry 452 triggers at least one output460 to raise the effective input impedance of the receiver amplifier vialine 462 and to enable the transmitter via line 464. When theillustrated circuit is in a transmit mode, the higher effective inputimpedance attenuates the input signal allowing the receiver amplifier tomonitor the transmission drive signal at the antenna element allowingself alignment of the circuit. The receiver input attenuation avoidsexcessive circuit loading when the transmit signal is received. Thus,with a larger receiver input impedance, the transmitter driver is ableto more efficiently drive the antenna element, and the amplifier isprotected from higher voltages provided by the transmitter driver. Thedriver circuit is capable of being monitored by the receiver circuitrythrough the larger impedance. When the RF input does not have a carrierof adequate RF drive level, the illustrated circuitry is in a receivemode, and the control circuitry triggers the at least one output tolower the effective input attenuation of the receiver path and disablesthe transmitter via line 464. The lower effective receiver attenuationenhances the efficiency of the antenna element to receive magneticsignals and provide a corresponding signal to the amplifier. When theillustrated circuitry is in a receive mode, the disabled transmitter isin a high impedance mode (e.g. open circuit) to avoid loading theantenna element when it is receiving the magnetically-transmittedsignals.

In various embodiments, the transmitter drive circuit 450 includes adifferential push-pull driver stage. When enabled, the driver stageconverts the transmit information into a differential output with lowoutput impedance at antenna resonant frequency providing maximum drivecurrent to the antenna element. In various embodiments, supply powersavings is achieved by disabling one of the output stages by using aground enable control signal 466. In various embodiments, the controlcircuitry or carrier detect circuitry triggers an output to provide theground enable control signal. Disabling one of the output stages allowsthe RF device level to be lowered by 6 dB.

FIG. 5 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter. In accordance with various embodiments of the presentsubject matter, the illustrated block diagram provides further detail tothe block diagram of FIG. 4. The illustrated antenna element 502includes a coil 512 and a tuning capacitor 508 in series with the coil512. The illustrated transmitter drive circuit 534 includes a push-pulldrive stage 550 and a control circuit 552 to control various operationsof the circuit based on detecting an RF carrier signal.

The illustrated amplifier 504 includes a voltage-driven amplifier 568.The voltage-driven operational amplifier has a very high inputimpedance. One of ordinary skill in the art will appreciate that theinput (R_(IN) and R_(S)) and feedback (R_(F)) resistors set theamplifier gain and the input impedance of the amplifier. The band-passresponse of the antenna circuit, and the amplifier's sensitivity tounwanted electric fields are capable of being modified by adjusting theinput impedance and/or the feedback impedance of the voltage-drivenoperational amplifier. Input and feedback elements are appropriatelyselected to function with a switch to appropriately adjust the band-passresponse and/or gain of the amplifier.

In various embodiments, the effective input impedance to thevoltage-driven amp is adjustable. In the illustrated figure, the inputimpedance network includes, for each input of the differential amplifier568, a first element or input resistor (R_(IN)) connected in series witha second element or shuntable resistor (R_(S)). An impedance shunt 548is formed across the shuntable resistors (R_(S)) such that the shuntableresistors (R_(S)) can be removed from the effective input impedance ofthe amplifier. Other impedance elements can be substituted to provide adesired switch-actuated adjustment to the band-pass and gain. In variousembodiments, the switches are formed by a transistor, such as a FETtransistor, coupled in parallel across the shuntable resistors (R_(S))with their gates operably connected to a control line from the carrierdetect control circuitry. An effective short is provided by providing apotential to the gate to turn on the transistor. This is illustrated inthe figures as a logic switches. When an RF carrier signal is detected,the carrier detect control circuitry opens the shunt (e.g. turns off thetransistor) such that the effective input impedance is larger toincrease the energizing current to the antenna element. The largereffective input resistance includes both R_(IN) and R_(S) for each inputof the differential amplifier. When an RF carrier signal is notdetected, the carrier detect control circuitry closes the shunt (e.g.turns on the transistor) such that the effective input impedance issmaller to increase the amplification of the magnetic field inducedcurrent received by the antenna element. The smaller effective inputresistance includes R_(IN) and excludes R_(S) for each input of thedifferential amplifier.

FIG. 6 illustrates a block diagram of a communication circuit to receiveand transmit signals, according to various embodiments of the presentsubject matter. In accordance with various embodiments of the presentsubject matter, the illustrated bock diagram provides further details tothe block diagram of FIG. 4. The illustrated amplifier includes acurrent-driven amp. The current-driven amp, also referred to as acurrent mode differential amplifier, has a very low input impedance. Theinput and feedback resistors set the amplifier gain and the inputimpedance. The band-pass response of the antenna circuit, and theamplifier's sensitivity to unwanted electric fields, are capable ofbeing modified by adjusting the input impedance and/or feedbackimpedance of the current-driven operational amplifier. Input andfeedback elements are appropriately selected to function with a switchto appropriately adjust the band-pass response and/or gain of theamplifier. In various embodiments, as discussed above with respect toFIG. 5 and the voltage-driven amp, the effective input impedance to thecurrent-driven amp is adjustable using the impedance shunt.

The present subject matter is capable of being incorporated in a varietyof near-field communication systems and technology that use suchnear-field communication systems such as hearing aids. For example, thepresent subject matter is capable of being used in hearing aids such asin-the-ear, half-shell and in-the-canal styles of hearing aids, as wellas for behind-the-ear hearing aids. Furthermore, one of ordinary skillin the art will understand, upon reading and comprehending thisdisclosure, the method aspects of the present subject matter using thefigures presented and discussed in detail above.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover adaptations or variations of the present subjectmatter. It is to be understood that the above description is intended tobe illustrative, and not restrictive. Combinations of the aboveembodiments, and other embodiments will be apparent to those of skill inthe art upon reviewing the above description. The scope of the presentsubject matter should be determined with reference to the appendedclaims, along with the full scope of legal equivalents to which suchclaims are entitled.

1. A method, comprising: receiving a first signal induced in an antennaelement, comprising: shielding the antenna element from electric fields;and controlling an input impedance switch to provide a differentialamplifier with a first input impedance; and transmitting a secondsignal, including: controlling the input impedance switch to provide thedifferential amplifier with second input impedance larger than the firstinput impedance; and driving the first signal through the antennaelement.
 2. The method of claim 1, wherein shielding the antenna elementfrom electric fields includes: distributing electrostatic charge acrossan electric field shield that extends along a length of the antennaelement to provide a common mode voltage at a first terminal and asecond terminal of the antenna element; and rejecting the common modevoltage using the differential amplifier.
 3. The method of claim 1,wherein shielding the antenna element from electric fields furthercomprises removing the electrostatic charge from the electric fieldshield using a reference potential.
 4. The method of claim 1, whereinshielding the antenna element from electric fields includes shieldingthe antenna element using an electrostatic conductor surrounding a coilof the antenna element.
 5. The method of claim 1, wherein shielding theantenna element from electric fields includes shielding the antennaelement using an electrostatic conductor strip extending a length of theantenna element.
 6. The method of claim 1, wherein controlling the inputimpedance switch includes selectively removing at least one element froman input impedance network for the differential amplifier.
 7. The methodof claim 6, wherein selectively removing the at least one element fromthe input impedance network for the differential amplifier includesshorting a series impedance to reduce the input impedance to the firstinput impedance, and removing a short from the series impedance toincrease the input impedance to the second input impedance.
 8. Themethod of claim 6, wherein selectively removing the at least one elementfrom the input impedance network for the differential amplifier includesshorting a parallel impedance to increase the input impedance to thesecond input impedance, and removing a short from the parallel impedanceto decrease the input impedance to the first input impedance.
 9. Themethod of claim 1, further comprising monitoring a transmission of thesecond signal using the differential amplifier and the second inputimpedance.
 10. The method of claim 1, wherein: transmitting the secondsignal includes enabling a driver to drive the second signal through theantenna element; and receiving the first signal includes disabling thedriver.
 11. The method of claim 1, further comprising: monitoring an RFinput to detect an RF carrier signal; transmitting the second signal inresponse to a detected RF carrier signal; and receiving the first signalwhen an RF carrier signal is not detected.
 12. A method, comprising:monitoring an RF input to detect an RF carrier signal; receiving a firstsignal induced in an antenna element when an RF carrier signal is notdetected, comprising: shielding the antenna element from electricfields; and controlling an input impedance switch to provide adifferential amplifier with a first input impedance; and transmitting asecond signal in response to a detected RF carrier signal, including:controlling the input impedance switch to provide the differentialamplifier with second input impedance larger than the first inputimpedance; and driving the first signal through the antenna element,wherein controlling the input impedance switch includes selectivelyremoving at least one element from an input impedance network for thedifferential amplifier.
 13. The method of claim 12, wherein shieldingthe antenna element from electric fields includes: distributingelectrostatic charge across an electric field shield that extends alonga length of the antenna element to provide a common mode voltage at afirst terminal and a second terminal of the antenna element; andrejecting the common mode voltage using the differential amplifier. 14.The method of claim 12, wherein shielding the antenna element fromelectric fields further comprises removing the electrostatic charge fromthe electric field shield using a reference potential.
 15. The method ofclaim 12, wherein controlling the input impedance switch includesselectively removing at least one element from an input impedancenetwork for the differential amplifier.
 16. The method of claim 12,wherein selectively removing the at least one element from the inputimpedance network for the differential amplifier includes shorting aseries impedance to reduce the input impedance to the first inputimpedance, and removing a short from the series impedance to increasethe input impedance to the second input impedance.
 17. The method ofclaim 12, wherein selectively removing the at least one element from theinput impedance network for the differential amplifier includes shortinga parallel impedance to increase the input impedance to the second inputimpedance, and removing a short from the parallel impedance to decreasethe input impedance to the first input impedance.
 18. A method,comprising: receiving a first signal induced in an antenna element,comprising: shielding the antenna element from electric fields using anelectrostatic conductor surrounding a coil of the antenna element or anelectrostatic conductor strip extending a length of the antenna element;and controlling an input impedance switch to provide a differentialamplifier with a first input impedance; transmitting a second signal,including: controlling the input impedance switch to provide thedifferential amplifier with second input impedance larger than the firstinput impedance; and driving the first signal through the antennaelement; and monitoring a transmission of the second signal using thedifferential amplifier and the second input impedance.
 19. The method ofclaim 18, wherein shielding the antenna element from electric fieldsfurther comprises removing the electrostatic charge from the electricfield shield using a reference potential.
 20. The method of claim 18,wherein shielding the antenna element from electric fields includesshielding the antenna element using an electrostatic conductorsurrounding a coil of the antenna element.